Narrowband detector

ABSTRACT

A narrowband signal detector has an adaptable filter and controller for controlling the centre frequency and the bandwidth of the filter, to track the narrowband signal. Better control of the filter can be achieved by basing the filter control on a comparison of output and input to the filter. The comparison gives a more direct measure of how well the filter is tracking the narrowband signal. In the case of a notch filter, if there is poor tracking. The control should be biased rapidly towards improved tracking speed. Otherwise, for good tracking, the control should be biased towards accuracy of tracking. This enables speed and accuracy of detection requirements to be met with less computational load. Applications include telecommunications signalling or data tone detection. Multiple narrowband signals can be detected by a cascade of filters.

FIELD OF THE INVENTION

The invention relates to detector arrangements including software andapparatus for detecting narrowband signals, to tone processingarrangements, to apparatus for a central office, to pseudo divisioncircuitry or software, to methods of using an output of sucharrangements for control or monitoring purposes, to methods oftransmitting voice or data using such arrangements, and to methods ofoffering a voice or data transmission service using the above.

BACKGROUND TO THE INVENTION

Detection of narrow-band signals such as tones is used in many fields,including imaging fields such as radar- and sonar-type signalprocessing, accoustic signal processing, including mechanical vibrationdetection, and telecommunications. Telecommunication equipment make aheavy use of tones for signaling purposes. Some examples of tonesignaling systems are DTMF (dual tone multi-frequency) and MF(multi-frequency) signaling. As a consequence, a variety of differentequipment needs to be able to detect and decode these tones to implementthe signaling system. Examples of the different equipment include forexample, switches for routing calls, modems for transmitting data,voicemail systems, and call-centre systems: Moreover, tone detectors arealso useful in other applications where tones could deteriorate theirperformance. An example of this is an Echo Canceller (EC). Narrow-bandsignals can corrupt the coefficients of the echo canceller and reducethe voice quality of the phone line. Therefore, these narrow-bandsignals have to be detected to enable the (Echo Canceller) to take theappropriate measure to protect itself. It may stop adaptation, stopcancellation, or take other measures.

FIG. 1 shows in schematic form a conventional application of anarrow-band detector within the context of telecommunications equipment.In this figure, a narrowband detector 10 is coupled to receive inputsignals representing telephone calls. The detector outputs signalsrepresenting frequencies, and powers of narrowband signals it detects,together with flags to indicate positive detection. A phase reversaldetector 20 is provided to detect any phase reversals, as specified inwell known signalling standards. The phase reversal detector is fed withthe same input signals, and additionally with the outputs of thenarrowband detector.

The outputs of the narrowband detector and the phase reversal detectorare fed to a tone decoder 30, which classifies the results into standardsignalling tones if they meet the standards in terms of duration,frequency and so on. The outputs of this unit are fed to units such asthe echo canceller 40, the routing switch 50, the call centre controller60, and the voicemail system 70. Some or all of these units aretypically located in a local exchange or central office 80, coupled tosubscribers 90 by subscriber lines 100, and coupled to other exchangesor central offices by trunk lines 110. In this figure, many otherelements or connections in the central office are omitted for the sakeof clarity.

The narrowband detector can be implemented in various ways. Fouriertransform based methods involve determining the frequency spectrum andlooking for peaks, and many varieties have been published. Another knowntechnique involves using an adaptive notch filter (ANF) to remove thenarrowband tone and measuring the residual or error signal. The centrefrequency is adapted continuously to track the tone, to minimise theresidual, using one of many possible control algorithms. The power andfrequency of the tone can be determined from the residual and the filtercentre frequency. This technique is used in various known narrowbanddetection techniques, including line enhancer, or sinusoidal detection,or cisoid detection, or frequency estimation techniques. An example ofan ANF for detecting narrowband signals such as sinusoids, is shown inIEEE transactions on circuits and systems-II Analog and digital signalprocessing volume 40 No. 7, July 1993, “On the adaptive lattice notchfilter for the detection of sinusoids”, by Cho et al.

A paper published by the IEEE in 1998, ref 0-7803-4428-6/98 “an adaptivehigh-order, notch filter using all-pass sections” by Torres et al, showsan example of an ANF. The adaptation rate is altered for non stationaryinput signals by altering the notch bandwidth dynamically. It statesthat the “notch bandwidth broadens when frequency variations aredetected, and narrows after convergence has been achieved.” This is inthe context of a filter using RPE (recursive prediction error) foradapting the centre frequency of the notch filter to converge on thenarrowband signal. Torres also shows tracking multiple narrowbandsignals, with cascaded notch filters. Each is independently controlled,it says “instead of using the overall output signal to adjust the filtercoefficients, this frequency decoupling property enables each section tobe independently minimised.”

Another example of an ANF type arrangement is shown in IEEE transactionson signal processing Vol 48, no 2 Feb. 2000 “Multiple fully adaptivenotch filter design based on allpass sections” by DeBrunner et al. Thearrangement implied by this paper is shown in FIG. 2. In this figure, aninput signal is fed to an adaptive notch filter 200. A filter controller210 is provided having an adaptive algorithm 220 for adapting a centrefrequency k of the ANF. The controller also has an adaptive algorithm230 for adapting a forgetting factor, and an adaptive algorithm 240 foradapting the bandwidth. The bandwidth, the centre frequency, and theforgetting factor are fed back to the ANF. The controller takes as itsinputs an error signal and an estimated error gradient output from theANF. It is also known to have arrangements in which the controller takesthe input signal as an approximation of an error gradient.

There are a number of problems with this approach. Firstly, when thereare two or more tones with closely spaced frequencies, the algorithm foradapting the ANF bandwidth cannot converge to either one. The error isminimised with the bandwidth being sufficiently wide to cover bothtones. However a much narrower bandwidth is desirable to give betteraccuracy of detected frequency and power.

Secondly, this method provides slow reconvergence for non stationarytones because the forgetting factor is tied to the bandwidth. This meansthat when the bandwidth is small and the forgetting factor large, andthe input tone frequency changes, the forgetting factor will only reducerelatively slowly. Thirdly, the method is relativelycomputation-intensive.

High speed detection of tones is desirable for many applications. Forexample, some telecommunication signalling tone standards specify thetone duration may be as little as 40 msecs, and may have a 20 msecinterruption. Also, echo cancellers and modems need to detect tones veryrapidly to ensure correct operation.

SUMMARY OF INVENTION

A first aspect of the invention provides a detector arrangement fordetecting a narrowband signal in an input signal, the detector having:

an adaptable filter coupled to the input signal and having a frequencyresponse with an adaptable centre frequency, and

a filter controller for controlling the filter, to track the narrowbandsignal, the controller being dependent on a comparison between the inputsignal and an output of the filter, the comparison indicating howclosely the filter is tracking the narrowband signal.

Better control of the filter can be achieved by basing the filtercontrol on this comparison. The comparison gives a more direct measureof how well the filter is tracking the narrowband signal. In the case ofa notch filter, if the output is large relative to the input, thisindicates poor tracking. The control should be biased rapidly towardsimproved tracking speed. Correspondingly, a small output relative to theinput, indicates good tracking and the control should be biased towardsaccuracy of tracking, at the expense of speed of tracking. Thisimprovement in turn enables critical speed and accuracy of detectionrequirements to be met with less computationally intensive algorithmsthan previously. It also enables better detection of multiple narrowbandsignals since there is little possibility of the above mentioned problemof convergence to a minimum error with the bandwidth covering twoclosely spaced narrowband signals. Using the input signal caneffectively prevent this.

Particularly for upgrading existing installations, such as centraloffices for telephone networks, the amount of processing power in termsof MIPs (millions of instructions per second) is typically a keylimitation, limiting the number of calls which can be handledsimultaneously. Hence any improvement in calculation efficiency willoften translate directly into an increase in call handling capacity, andtherefore an increase revenue generating capability.

A preferred way of implementing this feature is to base the control on aratio of a characteristic of the filter output as a proportion of acharacteristic of the input signal. Because the ratio (output/input ofthe ANFs) reflects directly if the ANFs are tracking the rightfrequencies or not, then the method can achieve a very fastre-convergence and at the same time converge very accurately. Anotherpreferred feature involves controlling the bandwidth according to thecomparison. The bandwidth is one of the best ways of biasing the controlbetween speed and accuracy.

Another preferred feature involves the filter having an adaptableforgetting factor, controlled by the controller. The forgetting factorenables the filter control to be further biased towards fast convergencewhen hunting for a signal, and biased towards accuracy at the expense ofconvergence speed, when convergence is achieved.

Another feature is to make the forgetting factor controllable on thebasis of the input signal, or the ratio of input and output signal. Thiscan give a further improvement for the same reasons set out above, thatthe ratio (output/input of the ANFs) reflects directly if the ANFs aretracking the right frequencies or not

Another preferred feature involves the controller using an adaptivealgorithm such as a recursive least squares (RLS) algorithm. Thisprovides a good balance of accuracy and speed of convergence, thoughwith a heavy computational load.

Preferably the arrangement is arranged to derive a bandwidth controlsignal from the input signal and the output of the filter, according tothe comparison and to a mapping defining how the bandwidth control isderived.

Preferably the mapping is a linear mapping with limiting of extremevalues. Preferably the detector arrangement is arranged to smooth thebandwidth control signal to reduce jitter. This can be done by timeaveraging, and can improve the balance between accuracy andresponsiveness.

Preferably the ratio is derived using a recursive pseudo divisionprocess. This can enable the computation load to be reducedconsiderably, since division operations are computationally intensive,and because the ratio needs to be calculated frequently. This saving ispossible because the ratio needs not to be calculated precisely inabsolute terms, provided trends are represented accurately.

Preferably the detector has a cascade of filters, to track multiplenarrowband signals simultaneously.

Preferably, the detector arrangement has multiple filters being arrangedin two or more rows of serially cascaded filters, the controller beingarranged to control the filters such that in each row, individualfilters track different ones of the narrowband signals, and in thedifferent rows, the same narrowband signals are tracked, but in adifferent order, the controller further being arranged to use errorgradients derived from the outputs of filters of one of the rows, anduse residual power outputs from one of the filters in each of the rows.This enables the disadvantages of triangular and series cascades to bereduced.

Another preferred feature involves processing detection of multiplenarrowband signals to remove double detections of the same signal, basedon frequency difference and power levels.

The arrangement may be in the form of software, recognising the value ofsoftware as a separately tradeable and deliverable item which can embodythe desired functions, and can become operational with relativelytrivial steps such as loading into standard hardware. Another aspect ofthe invention provides a tone processing system having a narrowbanddetector, and a tone decoder. Another aspect of the invention providesapparatus for a central office having such a tone processing system.

Another aspect provides software or circuitry arranged to derive anoutput representing a pseudo division of a signal representing anumerator by an input signal, using a continuous recursive process.Another aspect provides software or circuitry arranged to derive anoutput representing a pseudo division of a signal representing anumerator, by an input signal, by a continuous recursive process havingthe steps of:

multiplying the input by a previous output of the process, subtractingthe result from a constant, and multiplying the result of thesubtraction by the previous output. This enables the amount ofprocessing to be reduced considerably. For 32-bit precision for examplea division will require about 32“*” and 32“+” operations. In the case ofthe pseudo division, it can be reduced to: 2“*” and 2“+” operations, soit can be at least 16 times faster. It uses an approximation asexplained below, which is sufficiently accurate provided the input is aseries of values having only small changes between consecutive values.

Another aspect provides a detector arrangement for detecting multiplenarrowband signals in an input signal, the arrangement having:

multiple adaptable filters coupled to the input signal and havingfrequency responses with an adaptable centre frequency, and an adaptablebandwidth; and

a controller for controlling the centre frequency and the bandwidth ofrespective ones of the filters, based on outputs of the filters, totrack the narrowband signals,

the multiple filters being arranged in two or more rows of seriallycascaded filters, the controller being arranged to control the filterssuch that in each row, individual filters track different ones of thenarrowband signals, and in the different rows, the same narrowbandsignals are tracked, but in a different order, the controller furtherbeing arranged to use error gradients derived from the outputs offilters of one of the rows, and use residual power outputs from one ofthe filters in each of the rows.

Another aspect provides a detector arrangement for detecting multiplenarrowband signals in an input signal, the detector arrangement having:

multiple adaptable filters coupled to the input signal and havingfrequency responses with an adaptable centre frequency, and an adaptablebandwidth;

a controller for controlling the centre frequency and the bandwidth ofrespective ones of the filters, based on outputs of the filters, totrack the narrowband signals, and

an arrangement for removing duplicate detections of the same narrowbandsignal, based on frequencies and power levels of the detections.

Other aspects provide a method of using an output of the arrangement formonitoring or control purposes in voice or data processing equipment, amethod of transmitting signalling information or user data using thearrangement to detect the signalling information or user data, or amethod of offering a voice or data transmission service to subscribersusing the above-referenced apparatus for the central office. Theseaspects recognise the value of the enhanced narrowband detector inimproving various types of voice and data processing. Any of thepreferred features above may be combined with any of the aspects of theinvention, as would be apparent to those skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described in more detail to show byway of example how the invention can be implemented, with reference tothe drawings in which:

FIG. 1 shows a known telecommunications network including a narrowbanddetector,

FIG. 2 shows a known arrangement of an adaptive notch filter typenarrowband detector,

FIG. 3 shows a narrowband detector according to a first embodiment ofthe invention,

FIG. 4 shows a narrowband detector, according to another embodiment ofthe invention,

FIG. 5 shows part of the controller of FIG. 4,

FIG. 6 shows a cascade of filters according to another embodiment of theinvention,

FIG. 7 shows another cascade, according to another embodiment of theinvention,

FIG. 8 shows a pseudo division method according to another embodiment ofthe invention, and

FIG. 9 shows a flag processing arrangement according to anotherembodiment of the invention,

DETAILED DESCRIPTION

FIG. 3, Narrowband Detector

FIG. 3 shows a narrowband detector according to a first embodiment ofthe invention. It could be applied in the arrangement shown in FIG. 1 orin other applications. There is an ANF 300, which is fed by the inputsignal, and by one or more control signals from the controller whichuses an adaptive algorithm 310. These control signals include the centrefrequency. Other control signals may optionally be used, such asbandwidth. The ANF outputs a residual error power signal Eout which iscompared to the input signal by a comparator 305. The relationshipbetween these signals is used to control the ANF, either directly, or byinfluencing the controller. Other outputs of the ANF may also be used bythe controller. As ANFs are well known and can be implemented withoutdifficulty, there is no need to describe this function in more detail,and the reader is referred to appropriate text books or otherpublications, including those referenced above. In principle, the filtercan be a bandpass filter, though overall the system is easier toimplement if the filter is a notch filter.

Notably, the controller 310 also has the input signal as an input. Thisenables the advantages mentioned above in the summary of inventionsection, to be realised. Many variations or modifications can beenvisaged while still obtaining the benefit of using the input signal inthe adaptive algorithm for controlling the filter. In particular, manydifferent types of filter and many different types of adaptive controlalgorithm can be used.

FIGS. 4, 5 Controller for Adaptive Filter

FIG. 4 shows another embodiment. The ANF 360 is aranged to output aresidual power error signal (Eout or error), an error gradient, (Phi), aratio (Att) of residual power to input power, (Eout/Ein), estimatedpower (P) of a detected narrowband signal, and a flag to indicatepositive detection. The first three of these outputs are fed to thecontroller 420.

The controller 420 includes an RLS algorithm unit 370, for continuouslyupdating a centre frequency value k for the ANF. This value is fed backto the ANF via a delay element 410. The delay element is an optionalelement provided to ensure that a new updated value of k is not usedprematurely by the ANF. This is especially useful where the output isfed back to influence the next calculation. For example: at time n, theANF's filter the input with their current center frequency (k), thenfeed the results to the RLS which work out a next value for k This willbe used the next cycle (at time n+1) in the ANFs to filter the input andfeed the result to the RLS. The delay effectively providessynchronisation which is usually preferred to asynchronous typeoperation.

The controller also includes an active parameters section 380. Thissection derives suitable values for the bandwidth (alpha) of the ANF,and the forgetting factor (lambda) for the RLS algorithm. More detailsof this section are described below with reference to FIG. 5. Notablythe outputs of this active parameters section are dependent on the inputsignal (Ein), since the ratio of energies Eout/Ein is fed to the activeparameters section. This enables the advantages set out above. Theactive parameters section may be incorporated with the controller orwith the ANF as desired, without altering its function.

The RLS algorithm makes use of the error (Eout) and error gradient (Phi)output by the ANF. The frequency value k output by the RLS algorithm issmoothed by a time-averaging element 390 before being output to otherparts of the system, e.g. a tone decoder. A disabler 400 is provided todisable the operation of the RLS algorithm depending on the input level(Ein). One reason for doing this is to prevent corruption of internalcoefficients within the RLS algorithm when there is no input signal.This disabler is not essential, there are other ways of dealing withthis problem. It is important at a system level.

FIG. 5 shows more details of one possible implementation of the activeparameters section. Clearly other variations can be conceived. A lineartransformation of the ratio Att (Eout/Ein) is carried out by element 530to derive a value for the forgetting factor (lambda). Again thecomputation of the energies and the ratio can be implemented as part ofthe controller. A time averaging element 510 then smooths the output.Another linear transformation is carried out by element 540 to derive avalue for the filter bandwidth (Alpha). Another time averaging element520 is used to smooth this output

The forgetting factor is a value between 0 and 1 which influences howmuch the RLS algorithm to take into account past or historic values ofits inputs. Hence a low value gives faster convergence but loweraccuracy. A high value gives slow convergence but better accuracy. Moreaccuracy is desired when the ANF has converged on the narrowband signal.This is indicated when the ratio Att is low, since in this case most ofthe narrowband signal is being removed by the ANF. Hence the lineartransformation should have a negative slope. In principle it need not belinear, other more complex relationships could be used. It could even bea stepped or switched relationship.

A smooth relationship can help avoid an oscillatory response of thefilter, which would otherwise increase convergence delay and reduceaccuracy. Also, the relationship should be tailored to avoid extremevalues, which could cause difficulties. For example a value of zeromight cause divide by zero problems. A value of one might cause veryslow convergence or no convergence at all. Hence the output can beconstrained to be between 0.05 and 0.95 or similar values. Therelationship can be implemented using conventional methods, for exampleusing a multiplier for a linear relationship, and comparators for thelimits. Alternatively, look-up tables could be used.

For the transformation to generate the bandwidth, similar considerationsapply. A large bandwidth will give faster convergence, as there will bea steeper gradient in the frequency response away from the notch. Lowvalues of bandwidth will give slower convergence but more accuracy. Thisfollows because the frequency response of the ANF will have a sharperresponse at the peak. This in turn will lead to a greater error signalwhen the peak is not quite aligned with the corresponding peak in thefrequency spectrum of the narrowband signal. Hence the transformationshould be such that low values of Att lead to a low value for bandwidth.Again, a linear transformation is preferred, and extreme values shouldbe limited. If the bandwidth is too wide, for example an extreme valueof 1, this may cause numerical problems, such as overflow for fixedpoint calculations. An extreme value of zero may cause divide by zeroproblems. Also, If the bandwidth is made too narrow followingconvergence, then convergence may be lost.

The averaging of both outputs is useful to remove jitter. Jitter can bemore of a problem largely because of the new dependence on the inputsignal. There is usually little jitter on the output of the notchfilter. The severity of the problem depends on the characteristics ofthe input signal, and depending on the adaptive algorithm.

FIGS. 6, 7 Cascade of Filters

FIG. 6 shows a cascade of filters to enable multiple narrowband signalsto be tracked simultaneously. It is known from the Cho articlereferenced above to have a serial cascade of filters. The same articlealso shows an alternative type of cascade known as a triangular cascade.This involves a parallel set of rows of serial cascades, each cascadeending with a filter controlled so as to track a different narrowbandsignal from those tracked by the end filter of others of the cascades.The triangular structure is an optimised form of a square structure. Thecontroller may be implemented as separate controllers for each of thedifferent narrowband signals, independently optimising the tracking ofeach of the signals. Alternatively it can be implemented as a singlecontroller using an adaptive algorithm with several outputs. The singlecontroller can adapt to optimise the overall tracking of all thenarrowband signals.

A useful property of the serial cascade is that the filters cannotconverge to the same narrowband signal, because the first filter in thecascade removes the narrowband signal it is tracking. Hence the secondfilter will track a different narrowband signal. A disadvantage of theserial cascade compared to the triangular cascade is that theconvergence of the second filter is slower because it must wait untilthe first filter has converged. Also, the triangular scheme suffers fromconflict when there are fewer narrowband signals than there are ANFs, asthere is no inbuilt priority scheme to avoid multiple ANFs in the samecascade trying to track the same narrowband signal. This is the multipledetection problem.

In FIG. 6 a new arrangement is shown which uses features of botharrangements. There is both a serial cascade and a parallel arrangement.For the sake of clarity, a relatively simple arrangement having an orderof 2 is shown, suitable for tracking two narrowband signalssimultaneously. Of course, higher order arrangements with cascades of 3or more ANFs are feasible, following the same principles. Two ANFs, ANF1(550) and ANF2 (560) are coupled in series, the residual error from ANF1being used as the input to ANF2. Controller 1, (580) is used to controlthe centre frequency of ANF1. Controller ANF2 (590) is used to controlthe centre frequency of ANF2. It is feasible to replace the twocontrollers with separate algorithms with a single controller having analgorithm having two or more separate outputs to track two or morenarrowband signals. A third filter ANF3 (570) is shown coupled inparallel with ANF1. This filter is controlled by the same controller asis used for ANF2, and hence tracks the same narrowband signal as ANF2.

One key difference over the known arrangements is that the errorgradients from the two or more filters in the serial cascade are used bythe controllers, but the Eout/Ein ratios used by the controllers are nottaken from the serial cascade. Instead these ratios are taken from theparallel coupled filters. Hence controller 1 uses the error gradientoutput and error output of ANF1. Controller 2 uses the error gradientoutput by ANF 2 and the error output by ANF 3. This enables theconvergence speed of the parallel arrangement to be exploited, yetwithout losing the inherent prioritisation property of the serialcascade. This arrangement can be used with or without the abovedescribed feature of the controllers using the input signal, or ratio(Eout/Ein).

FIG. 7 shows another embodiment having a similar cascade arrangement andother features. In this figure, the controller or controllers are notshown, merely for the sake of clarity. 3 filters are shown, ANF 1-1, ANF1-2 and ANF 2-2, corresponding to ANF1, ANF2 and ANF3 of FIG. 6. Foreach filter, there are 3 inputs, an input signal, a frequency k and abandwidth (alpha), the latter two from the controller. Each filter is aconventional ANF having an error output (out) and another output (y)which is the bandpassed signal. The bandpassed signal is the output froma first stage of the ANF, the error output is output from a secondstage. The first stage involves bandpassing the input signal to pass theband around the centre frequency. The second stage involves removing or“notching” the centre frequency from the bandpassed signal.

The ultimate outputs at the right hand side of the figure are two errorgradients, one for each narrowband signal being tracked, for feeding tothe controller and power levels for each of the narrowband signals.Other outputs include signal energy levels at the input, and at theoutput of each of the filters, for feeding back to the controller or forcalculating the ratio Eout/Ein. In each case an energy level calculatingelement or circuit is shown for taking a continuous signal anddetermining an average energy level (illustrated as elements EnergyIn,EnergyInk1, EnergyInk2, and EnergyOut, respectively).

Outputs y from ANF1-1 and ANF 1-2 are fed via an optional multiplexer(“Mux”) to a series of elements for deriving an error gradient Phi. Thisis achieved in this implementation by a reordering operation to reorderfor convenience the multiplexed vector output by the multiplexer inelement 795 This is followed by stage labelled “AMP” to double thesignal value, and a delay stage (labelled “DEL”) for synchronisationpurposes as described above. The arrangement follows the same principleas described with reference to FIG. 6, in that the gradient iscalculated from the outputs of the serially connected filters.

Outputs y from ANF1-2 and ANF2-2 are used to calculate the output power,again following the principle explained above with reference to FIG. 6.The outputs y are fed to energy calculation elements (Energy1, Energy2respectively). The outputs of these elements are multiplied by powercorrection factors c1, c2, using multipliers Mult1 and Mult2respectively. This is to take into account filter characteristics, whichaffect the levels output by the filters. These can be determinedempirically, or calculated. They may depend on the frequency andbandwidth of the filters, in which case a range of correction factorscan be calculated, from which a current factor can be selected using thecurrent frequency and bandwidth values. The resulting power values areoutput for use by other parts of the system, optionally via amultiplexer (Mux1) and a delay element (Integer Delay 2). The delay isfor synchronisation purposes as described above, so that the updatedvalue is not used too soon by other parts of the system.

FIG. 8 Pseudo Division Method

FIG. 8 shows steps or elements for carrying out a pseudo divisionprocess, using multiply and add operations to carry out a divisionoperation in a recursive manner. This can usefully be applied tocalculating the ratio Eout/Ein, described above, or to many otherapplications. It uses an approximation which is sufficiently accurateprovided the inputs are continuous signals, not random values, so theresult of each successive calculation is not greatly different from thepreceding calculation. This means where B is an input signal and A=1/Bis the desired output, it can be seen that: $\begin{matrix}{A_{n + 1} = {1/B_{n + 1}}} & (1) \\{\quad{= {1/\left\lbrack {B_{n} \cdot \left( {1 + e_{n + 1}} \right)} \right\rbrack}}} & (2) \\{\quad{= {\left( {1 - {e_{n + 1 +}k\quad e^{2}\quad\ldots}}\quad \right)/B_{n}}}} & (3)\end{matrix}$and where e is small, ke² . . . can be ignored, so $\begin{matrix}{A_{n + 1} = {1/\left\lbrack {B_{n} \cdot \left( {1 + e_{n + 1}} \right)} \right\rbrack}} & (4) \\{\quad{= {\left( {1 - e_{n + 1}} \right)/B_{n}}}} & (5)\end{matrix}$in order to replace e_(n+1) in this equation, the following relation isused $\begin{matrix}{e_{n + 1} = {\left\lbrack {B_{n + 1} - B_{n}} \right\rbrack/B_{n}}} & (6) \\{\quad{= {\left\lbrack {A_{n} \cdot B_{n + 1}} \right\rbrack - 1}}} & (7)\end{matrix}$substituting this into equation (5) above gives: $\begin{matrix}{A_{n + 1} = {\left( {1 - \left\{ {\left\lbrack {A_{n} \cdot B_{n + 1}} \right\rbrack - 1} \right\}} \right)/B_{n}}} & (8) \\{\quad{= {A_{n}\left( {2 - \left\lbrack {B_{n + 1} \cdot A_{n}} \right\rbrack} \right)}}} & (9)\end{matrix}$This forms the basis of the arrangement shown in FIG. 8. Initially thedenominator labelled B_(n+1) is optionally added to a small constant c,using adder 800. This is simply to ensure the input is greater thanzero, to prevent problems from a division by zero. Next, a multiplier810 is used, to multiply the denominator B_(n+1) by a previous output,A_(n). This previous output is derived from the current output A_(n+1)using a delay element 820. The output of the multiplier is fed to anadder 830, via a limiting element labelled “Saturation 3” which acts tolimit the output of the multiplier, to limit the range, to ensure theapproximation in the algorithm is not invalidated. Next the adder isused to subtract the output of the limiting element from the value 2.Finally, the output of the adder is multiplied by the previous resultA_(n), derived from the current output A_(n+1) using the delay element.

This arrangement can result in significantly less processing than isused in a typical division operation of a microprocessor. Instead of thetypical 32 or more processor clock cycles used for a division operation,this uses only 4, ( two adds and two multiplications). This can bereduced to two cycles for devices with a multiply-accumulateinstruction. The saving is especially significant in applications suchas the narrowband detector described above, using the ratio Eout/Ein,since this value must be continuously updated.

Once the divide operation has been carried out, the result is multipliedwith the numerator, using multiplier 840. The numerator optionally has asmall constant c added using adder 850 for use if a correction isdesired. The output of this multiplier is passed through a limiter(labelled “Saturation2” in FIG. 8), to avoid extreme values.

FIG. 9 Flag Processing Arrangement

The narrowband detector can optionally have flag outputs to indicatedetection. They can be created by thresholding the ratio (Att) signalderived in the respective ANF or the controller. In any narrowbanddetector which is capable of tracking two or more narrowband signals, itis possible for the two or more filters to detect signals that are notintended to be separate narrowband signals. To be more certain ofcorrect detection, particularly in applications where the number ofconcurrent narrowband signals is not predetermined, it is useful to beable to suppress detection of one narrowband signal if it is too closein frequency to, and of much lower power than another detectednarrowband signal. FIG. 9 shows an arrangement to achieve this with aminimum of calculation overhead.

Two Att ratios, Att1 and Att2 are fed in from two ANFs. The respectivecentre frequencies k1 and k2 are fed in from the controllers. Thedifference in frequency values is determined by an adder 900, and theabsolute difference is compared to a threshold at element 910. Theoutput of this element is used to control switch1, which selects whetherboth flags are output, or whether one of the flags is suppressed. Whichof the flags is suppressed is determined by the remainder of thearrangement, based on inputs Att1 and Att2. Both inputs are firstcompared to thresholds t1 and t2. Provided the inputs are above therespective thresholds, a positive flag is generated by these thresholdelements. A comparator (labelled “minimum” determines which has thelower Att value, indicating a better convergence of the filter. Theoutput of this is used to control a switch (switch 2) which determineswhich of the flags is suppressed. In the example shown, one flag issuppressed by using multiplexers mux5 and mux6 to feed the two inputs ofswitch 2. Mux5 passes one flag and sets one at zero. Mux6 passes theother flag, and sets the first at zero.

This arrangement can be used in conjunction with the cascadearrangements of FIGS. 6 or 7, or with other arrangements for detectingmultiple narrowband signals.

Implementations for Telephone Call Processing

Any of the narrowband detector features described above can beimplemented as part of a tone processor in a central office as shown inFIG. 1. In this case the requirements for accuracy and speed ofdetection are especially valuable. Signalling tones such as DTMF tonesare widely used, and used for more different types of applications.Operators who offer telephone services to subscribers will often routecalls over equipment in central offices belonging to other operators.Quality of calls and whether they meet the well established standardsfor handling signalling tones is a major problem. Because the installedbase of central offices is so vast, the value in making improvements toexisting central offices as well as in new installations, is huge.Typically the tone processing sections and other modules in the centraloffice are implemented as software modules run on one or more DSPs(Digital Signal Processor). Accordingly, the narrowband detectorfeatures can be implemented in well known programming languages such asC or Ada, or others, as would be well known to those skilled in the art.The resulting code can be cross-compiled into a lower level languageappropriate to run on a DSP, such as the fixed or floating point typesmade by TI or Motorola or others, or on a general purposemicroprocessor, or any type of firmware, or programmable or fixedhardware, or any combination. The software can in principle beimplemented as instructions or as combinations of data, instructions,rules, objects and so on. Some features can in principle be implementedin dedicated hardware for greater speed of operation.

Concluding Remarks and Other Variations

Other variations will be apparent to those skilled in the art, withinthe scope of the claims. Although described with reference totelecommunications applications, other applications are intended to beencompasses by the claims. Although described with reference to a notchfilter, the aspects of the invention are clearly applicable to othertypes of filter. Although an RLS algorithm was used in the examplesdescribed, other types of algorithm can be used including other types ofLMS (least mean squares) algorithm. Although described using bandwidthand forgetting factor as the signals for biassing the control, either ofthese can be used individually, or other factors can be used, within thescope of the claims. Other applications include test equipment, datatransmission using modems, fax machines, monitoring, includingmechanical vibration monitoring, and similar uses.

Above has been described a narrowband signal detector has an adaptablefilter and a controller for controlling the centre frequency and thebandwidth of the filter, to track the narrowband signal. Better controlof the filter can be achieved by basing the filter control on acomparison of output and input to the filter. The comparison gives amore direct measure of how well the filter is tracking the narrowbandsignal. In the case of a notch filter, if there is poor tracking. Thecontrol should be biased rapidly towards improved tracking speed.Otherwise, for good tracking, the control should be biased towardsaccuracy of tracking. This enables speed and accuracy of detectionrequirements to be met with less computational load. Applicationsinclude telecommunications signalling or data tone detection. Multiplenarrowband signals can be detected by a cascade of filters.

1. A detector arrangement for detecting a narrowband signal in an inputsignal, the detector arrangement having: an adaptable filter coupled tothe input signal and having a frequency response with an adaptablecentre frequency, and a filter controller for controlling the filter, totrack the narrowband signal, the controller being dependent on arelationship between the input signal and an output of the filter, therelationship indicating how closely the filter is tracking thenarrowband signal.
 2. The detector arrangement of claim 1, thecomparison being a ratio of a characteristic of an output of the filteroutput as a proportion of a characteristic of the input signal.
 3. Thedetector arrangement of claim 1, the filter having an adaptablebandwidth, the controller being arranged to control a bandwidth of thefilter on the basis of the comparison.
 4. The detector arrangement ofclaim 1 the controller having an adaptable forgetting factor, theforgetting factor being adaptable according to the comparison.
 5. Thedetector arrangement of claim 1, the controller using an adaptivealgorithm.
 6. The detector arrangement of claim 5, the adaptivealgorithm being a recursive least squares (RLS) algorithm.
 7. Thedetector arrangement of claim 3, arranged to derive a bandwidth controlsignal from the input signal and the output of the filter, according tothe comparison and to a mapping defining how the bandwidth control isderived.
 8. The detector arrangement of claim 7, the mapping being alinear mapping with limiting of extreme values.
 9. The detectorarrangement of claim 7, arranged to smooth the bandwidth control signalto reduce jitter.
 10. The detector arrangement of claim 2, arranged toderive the ratio using a recursive pseudo division process.
 11. Thedetector arrangement of claim 1, additionally having multiple filters,and being arranged to track simultaneously multiple narrowband signalsin the input signal.
 12. The detector arrangement of claim 10, themultiple filters being arranged in two or more rows of serially cascadedfilters, the controller being arranged to control the filters such thatin each row, individual filters track different ones of the narrowbandsignals, and in the different rows, the same narrowband signals aretracked, but in a different order, the controller further being arrangedto use error gradients derived from the outputs of filters of one of therows, and use residual power outputs from one of the filters in each ofthe rows.
 13. The detector arrangement of claim 11, arranged to removeduplicate detections of the same narrowband signal by different ones ofthe filters, based on frequencies and power levels of the detections.14. The detector arrangement of claim 1 in the form of software.
 15. Thedetector arrangement of claim 1 in the form of apparatus.
 16. A toneprocessing arrangement having the narrowband detector arrangement ofclaim 1, and a tone decoder.
 17. Apparatus for a central office, theapparatus having an arrangement for routing voice or data signals, andhaving the tone processing arrangement of claim 16, coupled to therouting arrangement.
 18. Software or circuitry arranged to derive anoutput representing a pseudo division of a signal representing anumerator by an input signal, using a continuous recursive process. 19.Software or circuitry according to claim 18 wherein said continuousrecursive process comprises the steps of: multiplying the input signalby a previous output of the process, subtracting the result from aconstant, and multiplying the result of the subtraction by the previousoutput.
 20. A detector arrangement for detecting multiple narrowbandsignals in an input signal, the arrangement having: multiple adaptablefilters coupled to the input signal and having frequency responses withan adaptable centre frequency, and an adaptable bandwidth; and acontroller for controlling the centre frequency and the bandwidth ofrespective ones of the filters, based on outputs of the filters, totrack the narrowband signals, the multiple filters being arranged in twoor more rows of serially cascaded filters, the controller being arrangedto control the filters such that in each row, individual filters trackdifferent ones of the narrowband signals, and in the different rows, thesame narrowband signals are tracked, but in a different order, thecontroller further being arranged to use error gradients derived fromthe outputs of filters of one of the rows, and use residual poweroutputs from one of the filters in each of the rows.
 21. A detectorarrangement for detecting multiple narrowband signals in an inputsignal, the detector arrangement having: multiple adaptable filterscoupled to the input signal and having frequency responses with anadaptable centre frequency, and an adaptable bandwidth; a controller forcontrolling the centre frequency and the bandwidth of respective ones ofthe filters, based on outputs of the filters, to track the narrowbandsignals, and an arrangement for removing duplicate detections of thesame narrowband signal, based on frequencies and power levels of thedetections.
 22. A method of using an output of the arrangement of claim1 for monitoring or control purposes in voice or data processingequipment.
 23. A method of transmitting signalling information or userdata using the arrangement of claim 1 to detect the signallinginformation or user data.
 24. A method of offering a voice or datatransmission service to subscribers using the apparatus of claim 17.